Red Cell Indices: MCV, MCH, MCHC and RDW in a Blood Test

Four lines sit together on a blood count report and usually go unread: MCV, MCH, MCHC and RDW. Yet these, rather than haemoglobin itself, answer the central question in anaemia — which kind it is. Haemoglobin says anaemia exists. The indices say where to go next, and they do it in a minute at no extra cost.

What erythrocyte indices are and why they are calculated

Indices are not a separate test. In a single run the analyser measures the volume of each cell, the red cell count and haemoglobin, then derives MCH, MCHC and the haematocrit from those measurements. They therefore come with any ordinary complete blood count at no additional charge.

The first three describe an average red cell: how big it is and how densely it is filled. The fourth describes the diversity of the population — how alike the cells are. How these lines fit the rest of the report is shown in how to read a blood test.

MCV: the mean corpuscular volume and its normal range

MCV is the workhorse of the whole classification. It splits anaemia into three types, and that split sets the entire route of investigation:

• MCV below 80 fL — microcytic anaemia. Iron deficiency, thalassaemia, late-stage anaemia of chronic disease, more rarely sideroblastic anaemia and lead poisoning.
• MCV 80–100 fL — normocytic. Acute blood loss, haemolysis, anaemia of chronic disease, kidney failure, early iron deficiency.
• MCV above 100 fL — macrocytic. B12 and folate deficiency, alcohol, hypothyroidism, liver disease, drugs, myelodysplastic syndrome.

One trap here is serious. MCV is an average. When two deficiencies coexist — iron and B12 together, for instance — small and large cells average out and MCV reads normal despite marked anaemia. The only line that gives this away is a sharply raised RDW.

MCH and MCHC: how much haemoglobin sits in a cell

MCH almost duplicates MCV and moves with it: a smaller cell holds less haemoglobin. A fall in MCH is called hypochromia, and its practical meaning is the same — think iron, starting with ferritin and haemoglobin.

MCHC behaves differently, and that is exactly why it is useful. It barely changes in ordinary anaemia, because the cell shrinks along with its contents and the concentration stays put. So MCHC has a special role:

• MCHC above 360 g/L is rare. Hereditary spherocytosis, sickle cell disease, or far more often a laboratory artefact.
• MCHC below 300 g/L means marked hypochromia, nearly always severe iron deficiency.

Because it is so stable, MCHC works as an internal quality check: an implausible value points at the sample more often than at the person.

RDW and anisocytosis: why it shifts before MCV

RDW measures how much red cells differ in size. In healthy blood the cells are uniform and the spread is small. When the marrow starts working under shortage, it releases cells of differing calibre — small ones appear next to older normal ones, and the spread widens.

Before comparing your own figure with a reference range, check which line the report actually prints. Analysers report RDW two ways: RDW-CV as a percentage (11.5–14.5%) and RDW-SD in femtolitres (roughly 35–47 fL). A value such as 46 next to a range of "11.5–14.5%" looks alarming, yet it is simply the other unit.

The key practical point: RDW rises before MCV falls. In developing iron deficiency the sequence usually runs: ferritin empties, RDW rises, MCV and MCH fall, and only at the end does haemoglobin drop. An isolated high RDW with normal MCV and normal haemoglobin is therefore not a laboratory error but the earliest signal worth checking with ferritin.

The second use of RDW is separating diagnoses that would otherwise need extra tests:

The rule is not absolute, but it saves steps: a low MCV with a normal RDW and a family history points at thalassaemia rather than another course of iron.

The colour index: an obsolete measure and what replaced it

Older reports carried a colour index instead of MCH — a number near one, obtained by dividing haemoglobin by red cell count using a formula invented when direct measurement was impossible. Anaemia was graded from it as hypochromic, normochromic or hyperchromic.

The colour index is no longer used, and for good reasons: it is derived from two measurements rather than counted directly, loses accuracy at the extremes, and adds nothing beyond MCH. Automated analysers measure the volume of each cell directly, so MCV, MCH and MCHC replaced it entirely. If it still appears on a report, read it as a rough stand-in for MCH: below 0.85 means hypochromia, above 1.05 hyperchromia.

Low MCV: iron deficiency versus thalassaemia

This is the commonest fork in microcytic anaemia, and the indices settle it before any additional test.

For iron deficiency: high RDW, low or borderline ferritin, both MCV and MCH reduced, gradual onset, blood loss or dietary shortfall in the history. The full picture is in iron deficiency anaemia, and the code that most often appears on a discharge note is D50.9.

For thalassaemia: normal RDW despite a markedly low MCV, normal or high ferritin, a normal or raised red cell count alongside low haemoglobin, a stable picture over many years, southern ancestry. The key detail is that in thalassaemia the cells are numerous but small, while in iron deficiency they are usually few. That shows up in the RBC line beside haemoglobin — covered further in red cells and haematocrit.

The distinction is not academic. Iron supplements do not help a thalassaemia carrier, and taken long-term they harm: iron accumulates while the anaemia stays exactly where it was.

High MCV: B12, folate, alcohol and thyroid disease

Macrocytosis splits into two kinds, and the difference already shows on a smear.

Megaloblastic, from a shortage of vitamin B12 or folate. DNA synthesis stalls, the cell grows but fails to divide. Report signs: MCV often above 110 fL, high RDW, hypersegmented neutrophils, and frequently low white cells and platelets as well. The mechanism is set out in megaloblastic anaemia, and the neurological side in B12 deficiency, where nerve damage begins before the anaemia does.

Non-megaloblastic, MCV usually 100–110 fL: alcohol, liver disease, hypothyroidism, reticulocytosis after bleeding or haemolysis, drugs. A separate line is myelodysplastic syndrome in older people, where macrocytosis can be the first and only change for years.

A practical detail: any reticulocytosis lifts MCV, because young red cells are larger than mature ones. A high MCV straight after bleeding therefore does not mean B12 deficiency.

False results: cold agglutinins and lipaemia

Indices are calculated rather than observed, so pre-analytical problems can ruin them. Two cases turn up regularly and look equally recognisable.

Cold agglutinins. Red cells clump in the tube at room temperature and the analyser counts a clumped pair as one large cell. The result: red cell count too low, MCV sharply raised, MCHC above 380 g/L — a value that is physiologically impossible. Warming the sample to 37 °C and recounting fixes it.

Lipaemia — plasma made cloudy by fat after a heavy meal or with high triglycerides. The turbidity interferes with haemoglobin photometry, haemoglobin reads high, and MCH and MCHC follow it up while MCV stays normal. Hence the standard request for a fasting sample.

The general rule is simple: an MCHC above 380 g/L is nearly always an artefact, not a disease. Such a result means the test should be repeated, not that a rare diagnosis has been found. The same applies to indices that jump suddenly while every other line, platelets included, stays put.

What to do when red cell indices are abnormal

1. Start with MCV — it sets the type of anaemia and the whole route that follows.
2. Read RDW beside it — it separates deficiency anaemia from inherited anaemia and catches mixed deficiencies.
3. Check plausibility: MCHC above 380 g/L, or an MCV that jumps for no reason, means a repeat rather than a diagnosis.
4. Order ferritin for any microcytosis — the cheapest step with the largest yield.
5. For macrocytosis check B12, folate and TSH, and be honest about alcohol: it explains a sizeable share of cases.
6. Bring the whole report to the doctor, not one line: indices only read together.

Which tests actually make sense when anaemia is suspected is gathered in the anaemia test set. If the report is already in hand and the links between lines need to make sense before the appointment, that is what the lab report review does.

This article is informational. Diagnosis and treatment decisions belong to a doctor.